Childhood obesity is highly prevalent in the U.S. Family-based behavioral weight control treatment for younger overweight children (6-12 years old) has established efficacy, with sustained weight status improvements up to 10 years after treatment. Recent recommendations of the U.S. Preventive Services Task Force are that moderate- to high-intensity interventions (>25 contact hours) are needed to help overweight children achieve and sustain better weight control. However, the majority of health insurance coverage specifically disallows overweight/obesity treatment services or limits provider services far below recommended levels (e.g., up to 4 annual nutrition visits), making it difficult to sustai treatment programs. There are also considerably more overweight children in the U.S. alone, ~1 in 3 children, than professionals could possibly treat. A new model of delivering family-based pediatric overweight treatment that decreases costs, but sustains short- and long-term efficacy, is needed. One possible opportunity, to be developed and initially tested in the current proposal, is a peer intervention model in which families receiving professionally-delivered intervention subsequently provide intervention to other families. The proposal also examines whether peer interventionists' continued engagement in behavior change around eating and physical activity through providing intervention to others improves their own long-term efficacy. The proposed project begins with the evidence-based approach of professional interventionists delivering family-based behavioral weight control treatment to overweight children and their parent(s) (1st generation families; n=28). After receiving treatment, half of 1st generation families will be randomly assigned to serve as peer interventionists to other overweight children and parents (2nd generation families; n=28). The remaining 1st generation families will neither receive nor provide any additional treatment. This project aims to develop peer intervention training and delivery and to examine the feasibility and acceptability of the peer intervention delivery throughout the intervention. This project also aims to derive an estimate of the impact of peer intervention (receiving and providing) on child and parent weight outcomes, with assessments at the end of receiving treatment and 6 months later. This R21 application develops and begins to evaluate an innovative delivery strategy for family-based behavioral pediatric overweight treatment, with the potential for future comprehensive trials to examine longer term weight control efficacy, cost effectiveness, and dissemination.